Dihydroxynaphthalene may be prepared by oxidizing secondary alkyl-substituted naphthalenes, for example, diisopropylnaphthalene, to diisopropylnaphthalene dihydroperoxide (hereinafter sometimes called DHP for short) which is then subjected to acid decomposition with acid catalysts. Furthermore, diisopropylnaphthalene dicarbinol formed at the time of oxidation of diisopropylnaphthalene may be oxidized with hydrogen peroxide to DHP. This dihydroxynaphthalene is an industrially useful starting material for preparing various products, for example, synthetic resins, synthetic fibers, medicines, agricultural chemicals, dyes, etc.
U.S. Pat. No. 4,503,262 discloses to the effect that in a process for preparing DHP by oxidation of solutions of diisopropylnaphthalene in organic solvents with molecular oxygen in the presence of heavy metal catalysts, for example, cobalt salts of organic acids, the reaction rate, the yield and purity of the desired dihydroperoxide may be improved by using as the above-mentioned organic solvents, particularly aliphatic hydrocarbon solvents of 5-14 carbon atoms, for example, n-heptane. In the process as disclosed, however, the alleged improvement in the reaction rate or yield is not always to be satisfactory.
Furthermore, U.S. Pat. No. 2,751,418 discloses a process for preparing hydroperoxide by oxidation of secondary alkyl-substituted aromatic hydrocarbons with molecular oxygen in the presence of heavy metal catalysts such as palladium, platinum, osmium, iridium, ruthenium, rhodium, etc. In the process disclosed in said patent, however, the catalysts used are of so-called nonhomogeneous system, that is, they are those which have carried nobel metals on solid carriers, for example, alumina etc., or solid catalyst with a large catalyst surface area such as colloidal palladium, and these catalysts are dispersed in reaction solution. This patent fails to teach concretely in examples thereof the oxidation of secondary alkyl-substituted naphthalenes, though the patent discloses examples showing oxidation of alkylbenzene such as p-thymene, sec-butylbenzene, etc. In the oxidation reaction shown in the examples of this patent, such weak base as sodium hydrogene carbonate or sodium carbonate is used as the base, and accordingly said oxidation reaction is carried out only in the reaction solution of a low pH range.
British Patent No. 714,545 discloses to the effect that in the preparation of hydroperoxide by oxidation of secondary alkyl-substituted aromatic hydrocarbons with molecular oxygen, the oxidation ratio is improved when the oxidation reaction is carried out by using a reactor, the inner surface of which has been coated with copper, silver or gold, or stirring blades covered on the surface thereof with said metal. This patent, however, only discloses examples thereof wherein alkylbenzene such as cumene, diisopropylbenzene, etc. is oxidized with a reactor covered on the inner surface thereof with copper, and the effect as alleged to be obtainable thereby is found to be still insufficient.
British Patent No. 760,367 discloses a process for obtaining hydroperoxide by oxidation of cumene in the presence of copper formate or silver acetate, but the effect of the process is still insufficient.
In practicing the oxidation of secondary alkyl-substituted naphthalenes, the present inventors investigated the processes as disclosed in the above-mentioned patents and have ascertained that the oxidation reaction of the secondary alkyl-substituted naphthalenes cannot be carried out at a satisfactory rate even when any of the disclosed processes is employed therefor.
Further, it has been found that it was difficult to obtain hydroxynaphthalene in high purity as well as in high yields by subjecting a reaction mixture obtained by liquid phase oxidation of secondary alkyl-substituted naphthalenes with molecular oxygen to acid decomposition. Accordingly, it is of extremely high commercial value if hydroxynaphthalene can be obtained in high purity and in high yields from the above-mentioned oxidation reaction mixture.
Still further, Japanese Patent Laid-Open-to-Public Publication No. 282333/1986 discloses a process for preparing 2,6-dihydroxynaphthalene by one-state oxidation and acid decomposition of 2,6-diisopropylnaphthalene dicarbinol with hydrogen peroxide in acetonitrile or dioxane in the presence of inorganic acids. In this process, however, the starting substance to be subjected to the reaction is dicarbinol in its purified form free from impurities.